In recent years, the technology to transmit large amounts of information via optical communication has been developed reflecting the information age. At present, trunk lines of optical communication network have been installed with quartz fibers in the nation, and its extension to public facilities, factories, business offices, and further to private homes is being planned in the near future. For widespread availability of the optical fiber communication to every home, it is strongly desired to reduce its installation cost. There is a need for cost reduction in all of the optical fibers, communication modules, and optical parts. In order to meet this need, a system in which quartz fibers are not totally used but plastic optical fibers (POF) are partially used for short distance has emerged for local use among international lines, domestic trunk lines, urban communication network, office lines, and home lines. In other words, a large volume of data is transmitted to an urban relay station using quartz fibers, and the data is branched from there to transmit to every home using POF that is inexpensive and easy for installation and handling because of short distance.
Currently, the optical semiconductor sensor used for various optical devices includes materials such as starting with Si, Ge, Hg—Cd—Te system, In—Ga—As system, In—Ga—P system, and GaN.
Silicon (Si) is widely used for optical sensor because it is abundant in the earth, inexpensive, and low in environmental burden when disposed, its fabrication technology is well developed, and so on. Si has been playing an essential role in industrial fields since the twentieth century and has been widely used in accordance with the remarkable progress of technology. It has been used for not only optical sensor but also high-speed electric logic circuit, measurement sensor for various physical quantities, and switch.
However, Si can not be used for a light-receiving device for communication with quartz fibers. The wavelength of light for use in passing through quartz fibers is from 1.3 to 1.6 μm where transmission loss is minimized. Since the wavelength usable for Si ranges from visible light to 1.1 μm, transmission loss brought by using Si in quartz fiber communication is significant. In addition, Si has not a light-emitting property.
Accordingly, In—Ga—As system and In—Ga—P system having light-emitting and light-receiving properties are widely used for optical devices. These compound semiconductors utilize, however, elements such as As and In whose resource lives are very short in the earth. For example, although In is used for transparent electrodes (indium tin oxide, ITO) of plasma display as well, its resource life is expected to be about ten years from now, and that of As is also said to be at most that long. An element reserve is closely related to Clarke number. Elements with a larger Clarke number (light element with smaller element number) have been in contact with various living organisms on earth for long history and been incorporated into their bodies, and thus exhibit relatively high compatibility with living bodies. Moreover, these elements constitute substances that have lesser burden on the global environment. The above compound semiconductors are mainly composed of rare elements (In, As, Cd, Se, Hg) and many of these materials are high in toxicity and low in biocompatibility. The semiconductor materials to be frequently used in our society are desired to be made of elements with reduced burden on the environment from now on.
Beta-iron disilicide (β-FeSi2) is reassessed as a representative material composed of elements that are abundant in the earth and ecologically friendly to the global environment, and has become desired for a post Group III-V or Group II-VI compound semiconductor material. β-FeSi2 is a compound in which Fe and Si are combined in one to two ratio, and its crystal phase is stable up to 900 degrees C. or higher without any change. It is resistant to chemicals, and therefore resistant to both acid and alkaline agents, and its moisture resistance is also excellent. It has a physical property of a direct transition type semiconductor with an energy band gap of 0.85 eV, and is capable of light-emitting and light-receiving at a wavelength around 1.5 μm which lies in the region suitable for a communication part to be used for optical quartz fibers.
Furthermore, β-FeSi2 is brought into a good lattice matching with Si by choosing an appropriate surface and orientation from studying its crystal constant, and a good epitaxial film of β-FeSi2 can be grown on Si. When an optical device with β-FeSi2 is fabricated, a conventional Si process is applied as it is, and thus its fabrication is amenable to industry.
Examples of the studies on β-FeSi2 include light emission study in Non-patent document 1, light-receiving sensor study in Non-patent document 2, light emission studies in Non-patent documents 3 and 4, and further light-receiving sensor study in Non-patent document 5, and the like.
[Non-patent Document 1] M. A. Lourenco et al., Jpn. J. Appl. Phys., 40 (2001), 4041-4044
[Non-patent Document 2] Y. Maeda et al., Proc. Japan-UK Workshop on KANKYO-SEMICONDUCTORS, August. (2000), 29
[Non-patent Document 3] T. Suemasu et al., Appl. Phys. Lett., 79 (2001), 1804-1806.
[Non-patent Document 4] S. Chu et al., Jpn. J. Appl. Phys., 41 (2002), L1200-L1202.
[Non-patent Document 5] S. N. Wang, Proc. SPIE, 5065 (2003), 188
β-FeSi2 has a light-receiving sensitivity in an infrared region of wavelengths ranging from 1.1 to 1.6 μm and has no light sensitivity in a visible region of wavelengths ranging from 0.4 to 1.1 μm.